An increasing number of lighting solutions have been proposed to replace the ubiquitous linear fluorescent lamps used in 2′×2′ and 2′×4′ ceiling fixtures. Among the lamps being suggested to replace fluorescent lamps are elongated tubes containing various linearly distributed combinations of light emitting diodes (LED or LEDs). Other lamps include one or more two-dimensional spatial arrays of LEDs distributed throughout a rectangular fixture.
One particular arrangement of LEDs that has been used for general illumination is a plurality of spaced red- and white-light emitting LEDs. The blended light from all of the LEDs produces a quantity of white light with a higher color rendering index (CRI) due to the influence of the red-emitting LEDs. However, it is desirable for ceiling mounted fixtures to produce and evenly distributed diffuse light which is difficult to achieve with individually mounted LEDs alone.
Another approach is to use a linear arrangement or array of blue-emitting LEDs and a remote phosphor converter spaced at a distance from the LEDs which covers all of the LEDs, such as described in U.S. Pat. No. 7,618,157. The remote phosphor converter comprises a plastic material that has been embedded with a phosphor, in particular a yellow-emitting YAG:Ce phosphor. The blue light from the LED impinges upon remote converter which then coverts at least a portion of the blue light into light having a longer wavelength such as yellow. The combined effect is to produce a diffuse white light. In order to modify the spectrum to generate a higher CRI, a red-emitting phosphor can be mixed with the yellowing-emitting phosphor in the remote converter. However, because the remote converter covers the entire array, a much larger amount of phosphor must be used in comparison to using individual white-emitting LEDs. While this is less of a issue when using relatively inexpensive phosphors, red-emitting phosphors tend to be much more costly thereby making this approach less attractive for producing high CRI sources for areal lighting.